In order to cause electronic equipment to work, a stable direct current voltage is required for operating an IC or a transistor. Examples of systems of generating the stable direct current voltage, that is, a constant voltage include an AC (Alternating Current)-DC (Direct Current) system in which an input side (primary side) is a commercial alternating current power supply and a DC-DC system in which the input side (primary side) is a direct current power supply such as a battery.
Examples of a power supply apparatus which generates such a constant voltage include a switching power supply, which is a power supply apparatus adopting a switching system, and a power supply apparatus adopting a series system (dropper system, or linear system). In recent years, the mainstream is the switching power supply which has an advantage of being highly efficient and light.
The switching power supply has a great advantage to be able to use a coil, a transformer, or the like, which is small-sized, by high-frequency switching, but needs to perform switching of a high voltage at a high speed, so that switching noise is easily generated.
Particularly, in a flyback circuit system in which input and output are insulated by a transformer, when a primary-side switching element is brought into a turn-off state, switching is performed with a voltage obtained by superposing a flyback voltage on an input power supply voltage. Accordingly, noise resulting from high-voltage switching is easily generated.
Examples of a mode of generated noise include normal mode noise which is transmitted between power supply lines and common mode noise which is transmitted between each power supply line and the ground. The common mode noise is generated due to a stray capacitance which exists between a power supply line and the ground, asymmetry of transmission characteristics of power supply lines, or the like. It is generally difficult to quantitatively specify or estimate a generation point or generation level of noise, and, in some cases, it is difficult to reduce noise sufficiently only with a countermeasure by a filter which is arranged in an input portion of a switching power supply.
As a conventional technique by which common mode noise in a switching power supply circuit is reduced, a switching power supply circuit disclosed in PTL 1 and a noise reduction method for a switching circuit disclosed in PTL 2 are known, for example.
In a switching power supply circuit 100 disclosed in PTL 1, as illustrated in FIG. 8, a capacitance 101 is added between a primary-side voltage node 111 and a secondary-side node 112 of a transformer 110. Thereby, a common mode noise current whose phase is opposite to that of a common mode noise current which passes through a parasitic distributed capacitance of the transformer 110 is transmitted through the capacitance 101, resulting in that common mode noise currents at a ground point are offset.
Moreover, in the noise reduction method for a switching circuit 200 disclosed in PTL 2, as illustrated in FIG. 9, an auxiliary winding 212 which generates an electromotive voltage whose phase is opposite to that of a main switching winding 211 in a primary side of a transformer 210 is added to the primary side of the transformer 210.
Thereby, a stray capacitance Coa which is generated between a secondary-side winding 213 and the switching winding 211 in the primary side and which causes generation of noise is offset by an added stray capacitance Csa which is generated between the secondary-side winding 213 and the auxiliary winding 212 in the primary side and which has an opposite phase. As a result of this, common mode noise currents at a ground point are offset.